The above-cited applications describe a plasma reactor in which VHF plasma source power is applied by an overhead electrode while HF plasma bias power is coupled through a cathode in a pedestal supporting the semiconductor workpiece. The VHF overhead electrode tends to promote a center-high plasma ion density distribution. The electrical field produced by the bias power tends to be nonuniform, which also leads to a non-uniform plasma ion density distribution at the wafer or workpiece surface.
Uniform plasma ion density distribution is highly desirable, because it promotes uniform control of feature size, etch profile and etch rate across the entire workpiece surface, resulting in a wider process window. Therefore, features capable of correcting nonuniform plasma ion density radial distribution would be advantageous. In seeking such features, we have discovered that there are particular causes of non-uniform plasma ion density radial distribution that we propose to address in constructing such features. In particular, one cause arises from the presence of a center conductor extending vertically upward through the workpiece support. This center conductor conducts the HF plasma bias power to the workpiece. First, the center conductor affects the uniformity of power deposition by the overhead VHF electrode: it creates a non-uniform radial distribution of impedance in the ground return path of the VHF power through the wafer support pedestal. Secondly, the center conductor affects the radial distribution of the electrical field produced by the HF plasma bias power applied to the workpiece support. We propose to construct a feature that simultaneously addresses both the VHF ground path non-uniformities and the HF bias electrical field non-uniformities, or, in the absence of VHF source power, addresses the non-uniformities in the HF bias electrical field alone.